The present invention relates to a method for maintaining the cleanness within a vacuum chamber of a physical vapor deposition system, especially a method using plasma bum-in for maintaining the cleanness within a vacuum chamber of a physical vapor deposition system.
Physical vapor deposition method is one of the commonly used methods for depositing metal compound films in semiconductor processes. A metal target is located in a vacuum chamber of the physical vapor deposition system. After supplying a reactive gas and an inert gas into the vacuum chamber and producing a plasma from the reactive gas and the inert gas, the reactive gas derivatives in the plasma react with the metal target, forming metal compound depositing upon a processed wafer.
FIG.1 illustrates a vacuum chamber 1. Taking the physical vapor deposition of titanium nitride as an example, the metal target 4 is made of titanium. In typical, a process kit 9 is provided in the vacuum chamber 1 to prevent titanium and titanium compound from depositing upon the inner wall 15 of the vacuum chamber 1. During a process, a wafer is placed on the susceptor 10 and the reactive gas nitrogen and an inert gas (mostly argon) enter the vacuum chamber 1 through a gas inlet 12. The power supply 14 applies a negative potential on the metal target 4 which is in electrically conductive relationship with an end surface 2 of the vacuum chamber 1. The portions of the vacuum chamber 1 below the insulating material 13, including the opposite end surface 3, the susceptor 10, and the processed wafer are all in electrically conductive relationship and are grounded.
By this arrangement, an electric field is formed at the cavity 7 and a plasma is produced from the nitrogen and the inert gas. The ionized inert gas bombards the surface 5 of the titanium-made metal target to produce titanium particles. These titanium particles react with nitrogen atoms and nitrogen ions formed in the plasma to produce titanium nitride particles. Some of these titanium nitride particles deposit on the wafer, and some of them deposit on the side surface 6 of the titanium-made metal target and on the inner surface 91 of process kit 9. The particles formed in each process tend to stack on particles already attached to the side surface 6 and the inner surface 91. If the stacking process goes on over the time, nodules such as nodule 81 and nodule 82 will form. Since metal compounds, such as titanium nitride, are brittle in nature and nodules 81 and 82 are protrusive in shape and are not dense structurally, peeling particles from the nodules 81 or 82 are very likely to fall on the processed wafer due to the bombardment of the plasma, thus resulting in failure of the processed wafer.
The conventional plasma bum-in method for a vacuum chamber 1 is stated below in this paragraph. After completing a specific number of film deposition processes, an inert gas is supplied into the vacuum chamber 1. Then a plasma is produced from the inert gas to bombard the nodules into small particles and to bombard the metal target 4 for depositing a ductile metal film upon the brittle metal compound film, thereby maintaining the cleanness within the vacuum chamber 1.
In traditional plasma bum-in method, the operation pressure of the inert gas plasma for plasma bum-in process is identical to the operation pressure, 3.0 to 4.0 mtorr, for metal compound film deposition of the wafer process. Referring now to FIG. 3, the plasma distribution at this operation pressure, however, can not effectively bombard the nodule 82 on the side surface 6 of the metal target. The peeling of residue of nodule 82 will still occur and the particle contamination is not substantially improved. It is noted that after the conventional burn-in process, the titanium nitride film on the metal target surface has been removed by argon bombardment due to the plasma bum-in process and it is not desirable. Before performing further wafer processes, argon and nitrogen are supplied into the vacuum chamber 1 and a plasma is produced from argon and nitrogen to nitridize the bombarded surfaces of the titanium target so that a titanium nitride layer is formed on the titanium target. During this operation, the operation pressure of the plasma from argon and nitrogen is between 3.0 mtorr and 4.0 mtorr.
Therefore, a method is desirable for reducing the nodules on the side surface 6 of the metal target so that the number of particles falling upon the processed wafer is minimized and the yield is increased.
People skilled in this art know that when the pressure in the vacuum chamber is elevated, the ion density of the plasma increases but the ion energy decreases and the mean free path of the particles shortens. On the contrary, when the pressure in the vacuum chamber is lowered, the ion density decreases, and the ion energy and the mean free path of the ion increase. The mean free path represents the mean displacement of a particle before a collision with another particle in a vacuum chamber. It is generally believed that one should lower the pressure in the vacuum chamber to increase the mean free path when expecting a particle to enter a narrower space within the vacuum chamber. In this case, to meet the requirements of high integration density, process microminiaturization, current PVD design mostly aims at higher vacuum and large value of mean free path. By doing this, the etch of sub-micron scale to the wafer can be achieved since the earlier collision of the particles is avoided. In coordination with the utilization of ECR (Electron Cyclotron Resonance), ICP (Inductive Coupled Plasma), TCP (Transformer Coupled Plasma), etc., the ion energy and ion density can be further enhanced. For instance, the TCP9400 series poly etch systems produced by LAM Research Corp. are characterized by their utilization of TCP technology to reach high vacuum and high ion density. Before the emergence of the present invention, the trend of improvement of plasma bum-in operation is to achieve high vacuum and high ion density.
The present invention provides a plasma bum-in method for maintaining the cleanness within a vacuum chamber of a physical vapor deposition. The present invention produces a plasma with the operation pressure greater than 10.0 mtorr from an inert gas. The plasma distribution at this operation pressure reaches the side surface of the metal target. The plasma bombards the nodules within these sections and bombards the metal target to deposit ductile metal film upon the brittle metal compound film. In a preferred embodiment of the present invention, the preferable plasma operation pressure is between 15.0 to 30.0 mtorr.